Teaching science in English is increasingly common worldwide. This study explores the role of language in science meaning-making during classroom interactions by drawing on video-recorded classroom observations of four biology lessons in secondary English medium instruction (EMI) settings in Hong Kong. Informed by systemic functional linguistics (SFL), we integrate classroom interaction analysis with Lemke's approach to thematic analysis. The results reveal similar interactional patterns across full- and partial-EMI science classrooms, characterised by a lack of interactivity and authoritative teacher behaviour in selecting student responses to engage with. During extended initiation-response-feedback exchanges, teachers' feedback on students' responses appears to underscore the importance of teachers' modified input - specifically input emphasising semantic relations within the field of biology - in developing students' content knowledge and language skills. The implications of the findings for applying SFL pedagogies in EMI science classrooms to improve teacher-student interaction are discussed. Copyright © 2025 De Gruyter .

Against the backdrop of internationalisation of education and the emergence of English as a global language, medium of instruction (MOI) has long been a controversial issue globally. The present study examined the relationships between secondary-level MOI (English, EMI; Chinese, CMI) and learners' tertiary-level academic achievement and perceptions. 434 tertiary-level business, science, and social science students in Hong Kong completed a survey and 18 of them also attended an interview. An overall significant difference was found in tertiary-level academic achievement between the EMI and CMI students. Significant differences were also found in their self-rated performance in and preferences for some but not all kinds of assessments. When analysed by tertiary-level disciplines, business students showed no differences while science students showed some, and social science students showed the greatest differences between the CMI and EMI groups. The interviewees highlighted academic and psychological challenges facing CMI students. Implications for educators and researchers are discussed. Copyright © 2023 Taylor & Francis.